1. Field of the Invention
This invention relates to an ultrasonic system for measuring the level of a liquid in a container, such as for instance, the level of reactor coolant in the pipes of a nuclear reactor.
2. Background Information
There are many applications where it is desirable to measure the level of liquid in a container, such as a pipe, without penetrating the liquid barrier. One such application is a nuclear power plant where there is a need to accurately know the water level in the main coolant pipes during maintenance operations when water is being circulated for residual heat removal (RHR). The normal water level under these conditions is above the center of the horizontal pipes, but below the full-pipe level. If the level drops too low, vortexing can occur causing air entrainment with the potential for air binding of the pump. High water levels during maintenance activities on steam generators or reactor pump seals, on the other hand, have resulted in reactor coolant spills and personnel contamination.
A commonly used method of measuring water level in the coolant pipes is to observe the level on a flexible plastic tube connected to the coolant pipe. This requires penetration of the pipe which has the potential for leaks and loss-of-residual-heat-removal events.
Assignee of the subject invention has developed a non-intrusive ultrasonic level measurement system which does not require penetration of the pipe. While other ultrasonic liquid level measurement systems exist, there are particular problems in their application to nuclear power plants. First, it is desired that the transducers remain in place during plant operation in order to eliminate the cost AND personnel radiation exposure of repeated installations. However, this exposes the transducers to temperatures up to 650.degree. F. and high radiation levels for long periods of time. Another difficulty in this application is that most plants have centrifugally cast stainless steel pipe, which strongly attenuates ultrasonic waves. Also typical pipe dimensions are 29 inch to 50 inch inside diameter and about 21/2 inches in wall thickness.
In the existing ultrasonic level measurement system, a transducer placed on the bottom of the horizontal pipe launches an acoustic wave which passes upward through the pipe into the water where it is reflected by the water surface and returns back through the wall to the transducer. The total travel time of the pulse reflected by the water surface is used to calculate water level. An echo is also produced at the pipe's inner surface because of the acoustic impedance mismatch between stainless steel and water. This echo reverberates within the pipe wall with the reverberation decaying with time. A threshold is established to distinguish the surface reflection signal from the reverberations. This threshold must be set low enough to detect high water level signals which are attenuated by their longer travel through the water, resulting in the reverberation signals being greater than the threshold for low level signals. The threshold is set by assuming a high threshold and then successively lowering the threshold until a selected number of reflected pulses out of a given number of trigger pulses exceed the threshold. If a prescribed number of these reflected pulses are within a predetermined time range, the last value of the threshold is reduced by a given percent with the result used as the threshold for determining water level. This system could not monitor the required lower water levels because of reverberation.
There is a need for an improved ultrasonic level measuring system which is more reliable and can accurately measure low liquid levels.